1. Field of the Invention
The present invention relates to a laminated notch filter mainly used in a high frequency appliance such as a mobile communication apparatus, and a cellular phone using the same.
2. Related Art
Recently, laminated notch filters have become used in various wireless apparatus, especially in cellular phones. Referring now to the drawing, an example of conventional laminated notch filter is explained.
FIG. 7 is an equivalent circuit diagram of a conventional laminated notch filter. In FIG. 7, the notch filter comprises two input/output terminals 1, two coupling capacitors 2 and two quarter wavelength resonators 3. One end of each coupling capacitor 2 is coupled to open ends of the quarter wavelength resonators 3. The two coupling capacitors 2 are connected almost in cascade through an inter-stage coupling line 4 which has a length of a quarter wavelength. The quarter wavelength resonators 3 can be mutually coupled by electromagnetic coupling (The electromagnetic coupling is symbolically described as xe2x80x9cCxe2x80x9d in the drawing).
The operation of the laminated notch filter having such a structure is explained below.
First, since the input/output terminals 1 are connected through the inter-stage coupling line 4, signals of ordinary frequency are transmitted without being affected. That is, hardly any insertion loss occurs. By contrast, at a specific frequency at which series resonance occurs in a series circuit of the coupling capacitors 2 and the quarter wavelength resonators 3, a signal to be transmitted is connected to the ground with nearly zero impedance, and is thus hardly transmitted. That is, at the frequency of series resonance, ideally, the amount of attenuation is infinite (for example, see Japanese Patent Laid-Open Publication No. 10-178302).
However, this ideal is realized only when the electromagnetic coupling between the resonators can be ignored, for example, in case that coaxial resonators are used, or when strip line resonators are spaced by a sufficient distance.
Generally, when a length of the inter-stage coupling line 4 is as short as a quarter wavelength, the electromagnetic coupling C occurs between the resonators 3. The electromagnetic coupling C between resonators 3 results in deterioration of the attenuation amount according to the intensity of the coupling, as shown in FIG. 8 (that is, attenuation amount diminishes as the coupling amount increases). Thus, for the structure in which the electromagnetic coupling C between resonators 3 can not be ignored, the attenuation amount varies. Therefore, there is a problem in that a small size notch filter with favorable attenuation characteristic cannot be obtained.
In the light of such problems, the aim of the present invention is to provide a small-sized laminated notch filter having a favorable attenuation characteristic even though the electromagnetic coupling between resonators can not be ignored.
In a first aspect of the invention, a notch filter is made up of laminated dielectric, and attenuates a specific frequency of a signal to be transmitted. The notch filter comprises two terminals to input or output a signal to be transmitted, a line connected between those terminals, two resonators each having one end connected to ground and an other end, two first coupling capacitors, and a second coupling capacitor. Each end of the line is connected to another end of each resonator through the first capacitor. The second capacitor couples the two resonators to each other.
In such a first notch filter, an attenuation frequency of the notch filter may be equal to an anti-resonance frequency of a parallel circuit of the second capacitor and an equivalent circuit which is obtained by taking, as a circuit, the electromagnetic coupling between the resonators.
In a second aspect of the invention, a notch filter is made up of laminated dielectric, and attenuates a specific frequency of a signal to be transmitted. The notch filter comprises two terminals to input or output a signal to be transmitted, a line connected between those terminals, two stepped impedance resonators each having a low impedance portion end a high impedance portion, and two coupling capacitors. Electromagnetic coupling in the two stepped impedance resonators is adjusted by controlling electromagnetic coupling between the low impedance portions and electromagnetic coupling between the high impedance portions, respectively.
In such a second notch filter, the stepped impedance resonators may be controlled such that an attenuation frequency of the notch filter is equal to an anti-resonance frequency of an equivalent circuit which is obtained by taking, as a circuit, electromagnetic coupling between the low impedance portions and electromagnetic coupling between the high impedance portions in the resonators.
In a third aspect of the invention, a notch filter is made up of laminated dielectric, and attenuates a specific frequency of a signal to be transmitted. The notch filter comprises two terminals for input or output of the signal to be transmitted, a line connected between those terminals, length of the line being shorter than an eighth of a wavelength of the signal to be transmitted, two resonators each having one end connected to ground and another end, and two coupling capacitors. Each end of the line is connected to another end of a resonator through the coupling capacitor.
In a fourth aspect of the invention, a cellular phone comprises a circuit for amplifying a signal and the notch filter according the present invention. The filter attenuates a specific frequency of the signal output from or input to the circuit.
According to the invention, the laminated notch filter of small size and with large attenuation can be obtained. Further, by using the notch filter according to the invention, a cellular phone having small size and high performance can be obtained.